Wide band radio frequency choke coil



1954 H. E. GOLDSTINE WIDE BAND RADIO FREQUENCY CHOKE COIL 2 Sheets-Sheet1 Filed July 19, 1951 0 40. an 12.0 100 20.0 z4.0 20.0 52.0 500FREQUENCY /A/ MEG/V6255 HBIIaQJ E. file R H m m w 0 0 3 m Oct. 19, 1954H. E. GOLDSTINE 2,692,372

I WIDE BAND RADIO FREQUENCY CHOKE CQIL Filed July 19, 1951 Q 2Sheets-Sheet 2 "5/11; R I 5 flall aaol sm INVENTO ATTORNEY Patented Oct.19, 1954 WIDE BAND RADIO FREQUENCY CHOKE COIL Hallan Eugene Goldstine,Port J eiferson Station,

Y., assignor to Radio Corporation of America, a corporation of DelawareApplication July 19, 1951, Serial No. 237,646

9 Claims.

This invention relates to radio frequency choke coils, and particularlyto radio frequency choke coils useful over a wide frequency range.

A conventional radio frequency choke coil comprising a single windinghas a highly uneven impedance characteristic over a wide frequencyrange, with anti-resonant peaks spaced from resonant dips occurring atdifierent frequencies within the range. As is well known in the art, thecapacitance between adjacent turns of a winding acting together with theseries inductance of the winding produces anti-resonant peaks andresonant dips in the impedance characteristic of a single wound coil,causing the impedance of the coil to vary widely over a frequency rangeof any considerable extent. This characteristic is highly undesirable intransmitters which must operate on more than one frequency.

It is an object of this invention to provide a radio frequency chokecoil useful over a wide frequency range without self-resonant dips inthe impedance characteristic.

Another object of the invention is to provide a novel choke coilconstruction having a low resistance to direct current and low frequencycomponents, and which has a relatively high and substantially uniformimpedance over the entire wide high frequency band which it is desiredto attenuate.

A further object of this invention is to provide a radio frequency chokecoil having two closely coupled windings in which one winding is of lowresistance wire and the other winding is of relatively high resistancewire compared to the first winding.

Briefly, in accordance with the present invention, there is provided achoke coil having a winding of low resistance wire which is adapted tobe electrically. connected in series in the utilization circuit, and asecond winding very closely coupled to the first winding but made ofhigh resistance wire to dissipate the high frequency components andreduce the peaks of selfresonance of the choke coil. The coil of theinvention may be considered to be a bifilar winding constituting atwo-terminal network in which one of the windings is of low resistancewire and connected at both ends to terminals, and the other winding isof high resistance wire closely coupled to the low resistance windingbut electrically connected at only one end of the high resistancewinding.

A more complete understanding of the operation and features of theinvention, together with additional objects thereof, may be gained froma reading of the following description in connection with theaccompanying drawings, in which:

Fig. 1 shows, schematically, the electrical circuit diagram of the chokecoil of the present invention;

Fig. 2 illustrates an embodiment of a choke coil, partly in crosssection, in accordance with this invention;

Fig. 3 shows an impedance versus frequency curve for a choke coil of thepresent invention compared with a choke coil without the high resistancewinding;

Fig. 4 illustrates a cross-sectional view of a second type of chokeembodying the principles of this invention;

Fig. 5 illustrates, partly in cross section, another form of theinvention, and

Fig. 6 is a circuit diagram of a push-pull amplifier showing, by way ofexample, locations where the choke coils of the present invention may beused.

Referring now to Fig. 1 of the drawings, which is a schematicrepresentation of a choke coil according to this invention, there isshown a low resistance winding II which is provided at each end withterminals [3, l4. Wound on the same core and closely coupled to the lowresistance winding I l is a high resistance winding I 5, which may beconnected to one of the terminals [4 of the choke coil. The closecoupling between the high resistance winding l5 and the low resistanceWinding Il may be obtained by winding the two conductors side by side ona single coil form in insulated relationship, or one winding may bewound in a single layer and the other wound on top of the first inanother layer, separated only by a thin insulating covering.

Close coupling may also be attained with pie windings. A pie Winding isone composed of many turns which are built upon one another to form astructure Which has an outside diameter several times the width of thecoil winding, and results in a flat disc shape coil. Individual coils ofthis fiat construction are termed pies. A construction utilizing piewindings employs a low resistance winding ll, each pie section of whichis in very close physical relationship and tightly coupled to adjacentpie-shaped sections of a high resistance winding [5, and is describedbelow in connection with Fig. 4.

In the choke coil construction shown in Fig. 1, a very low resistance isofiered to currents of low frequency and especially to direct currentthrough the low resistance winding II. On the other hand, by virtue ofthe tight coupling between the low resistance winding I I and the highresistance winding [5, high frequency currents are coupled into the highresistance winding I and are dissipated in the high resistance winding.

In Fig. 2 there is shown a choke coil in accordance with the principlesset forth above with respect to Fig. 1 having a high resistance windingI5 wound in a single layer and a low resistance winding II in a secondlayer over the high resistance winding I5 but insulated therefrom by athin insulating material II. It will be noted that both of the windingsI i, I5 are connected to. one of the terminals I4 at one end whereasonly the low resistance winding I I is connected to the terminal 13 atthe other end. In this form of the invention, the high resistancewinding I5 is not terminated at one end. A coil form or core I5 isprovided which, depending upon the band of frequencies for which thechoke coil is designed, may be of a plastic material such as Bakelite orpolystyrene, either hollow or solid. If the coil form is hollow, amagnetic core material may be inserted therein. In one embodiment of theinvention successfully tried out in practice, a choke coil which wasfound to work satisfactorily over the band of frequencies from 5 to 3,0megacycles, a core of insulating material inch in diameter and i longwas first wound with Nichrome wire of No. 40 gauge, 0.003 inch diameter,40 turns, per inch and 12% turns total. The Nichrome wire had a D. C.resistance of 1400 ohms. A thin insulating sleeve of low-loss materialwas placed overthe Nichrome winding and a winding of copper wi Of No. 28gauge, 0.015 inch diameter, was wound 66 turns per inch, 198 turnstotal. over the Nichrome winding and insulating sleeve. The copper wirehad a D. C. resistance of 2 ohms. Both ends of the low-resistance,copper winding were connected to the terminals it, I4 while only one endof the Nichrome winding was, connected to a terminal Hi, the other endbeing left unterminated.

The high resistance winding I5 may be of other metals besides Nichrome.For example, climax metal, chromax, constantin, German silver, manganin,or some other alloys of high specific re sistance can be used. Nichromeis preferred since it has a higher specific resistance than any of the bre.

In Fig. 3 there is shown an impedance versus frequency curve for a,choke coil in accordance with the teachings of the. present inventioncompared with a choke coil of conventional form without a highresistance winding. The impedance of a wide band radio frequency chokecoil in accordance with this invention is shown by the solid line 243,while the impedance of a radio frequency choke coil with a single layerconventional winding is indicated by a dash line 2i. Both curves extendover a frequency range from 5 to to megacycles.

It will be noted that the choke coil of conventional construction gave anearly zero impedance in two places 23, 25 at frequencies of 24.6 and 37megacycles and had two very high impedance points at placescorresponding to approximately and 26 megacycles. These high impedancepoints are anti-resonant peaks which are not shown in the drawing inorder to conserve space. On the other hand, the biiilar coil of thepresent invention gave a characteristic which varied from a minimum ofabout 4,000 ohms to a maximum of 8,500 ohms over the entire frequencyband from 5 to 30 megacycles. No points of zero or very low impedancewere found in the choke coil of this invention, and the anti-resonantpeaks of very high impedance are greatly reduced. The resultantimpedance characteristic is smoother over the wide range of frequenciestested.

The choke coils used for comparison in Fig. 3 had similar low resistancewindings of 198 turns of No. 28 gauge copper wire on a inch polystyrenerod. The double wound choke coil whose curve is indicated by the solidline 2E3 had a high resistance Nichrome winding closely coupled to thelow resistance winding like that described in connection with Fig. 2above.

Series resonance points occurring in choke coils, such as those shown at23, 25 in Fig. 3, produce impedances which are so low that it takes onlya small amount of radio frequency voltage at those frequencies ofresonance, whether it be the fundamental or a harmonic of the signalingfrequency or current arising from spurious oscillations within the radiofrequency equipment, to produce currents which are large enough to burnout the low resistance winding and destroy the choke coil or causeimproper operation of the radio frequency equipment.

In Fig. 4 a cross-sectional view of a multisection pie-wound choke coilis shown utilizing the principles of this invention. A low resistancewinding H is composed of several sections of a pie shape universalwinding. Such a universal winding is made by winding the wire upon acylindrical form accompanied by an oscillatory motion with respect tothe cylinder. The resulting coil structure is self-supporting and can bemade very narrow with respect to the outside diameter. A multi-sectionedpie-shape high resistance winding i5 is wound on the same form and theindividual pies of the high resistance winding are inter-leaved with thepics of the low resistance winding II. The spacing between adjacent piesections is made small to increase the coeiiicient of coupling betweenthe low resistance winding I I and the high resistance winding I5. Theindividual pics of the low resistance winding I I are connected inseries and both ends are brought out to terminals in the usual manner asshown in Figs. 2 and 5. A coil form I9 provides the support for theentire assembly and may be of an insulating material, either solid orhollow. The coil form It} is preferably a hollow tube to allow a core ofmagnetic material to be inserted therein.

An advantage which may be pointed out with respect to the multi-section,inter-leaved pie wound choke coil is that a greater number of turns ofwire can be put into a very small space, and a reasonable value of thecoefficient of coupling can still be obtained. It may be noted at thispoint that the spacing between the individual pics in the design shownin Fig. 4 is much smaller than that usually encountered in either radiofrequency chokes wound with multiple pie sections on a single core orcoil form I9. The reason for the narrow spacing between the pieshapedsections is to increase the coefiicient of coupling between the lowresistance winding II and the high resistance winding I5. Further, thepies containing the high resistance winding I5 may have a great manymore turns in each section than those pies of the low resistance windingii. In a choke coil according to this invention having the form shown inFig. 4, for greater current carrying capacity and lower direct currentresistance, a low resistance winding may be made of wire of a largerdiameter than that of the high resistance winding l5.

Fig. 5 shows, partly in cross-section, a bifilar wound choke coil inaccordance with the present invention in which the high and lowresistance windings are in side-by-side relationship. The low resistancewinding 1 l, which i preferably of copper or similar low resistanceconductive wire, is in insulated relationship with, but tightly coupledto, the high resistance winding [5. Both windings are wound on a coilform ll; of insulating material, which in this illustration is shown asbeing tubular. Two terminals l3, M are provided on the coil form. Onlythe low resistance winding II is connected to one of the terminals IS;the high resistance winding [5 may be left unterminated at one end. Bothhigh and low resistance windings ll and I 5 are connected to the otherterminal M. A multifilar winding having more than one high resistancewinding l5 may also be utilized to carry out the present invention. Insuch an arrangement, the impedance of the separate high resistancewindings will appear in parallel, and the damping effect of the highresistance winding will be reduced.

A core of magnetic material may be inserted into the interior of thecoil form IS. The advantage of inserting a core of magnetic materialinto the coil form I9 is to effectively lengthen the winding for allfrequencies of alternating current. Greater choking action may thus beobtained by utilizing a core of magnetic material.

In Fig. 6 there is shown by way of example a push-pull amplifier circuitutilizing the wide frequency band choke coils of the present invention.Two vacuum tubes, VI and V2, which for the purpose of illustration areshown as tetrodes, have their grids 21 and 29 fed in phase oppositionfrom a pair of input terminals 3|. The anodes 33, 35 of the vacuum tubesVI and V2 are coupled together in phase opposition or push-pull relationby means of an output tank circuit having a center-tapped inductor 31.Anode voltage is supplied from the positive terminal 13+ of a source ofunidirectional potential, not shown, to the center point of the inductor3'! through a wide band choke 4| in accordance with this invention. Sucha choke 4! may take the form of Fig. 2, Fig. 4, or Fig. 5. Voltage forthe screen electrodes 43, 45 of vacuum tubes VI and V2 respectively mayalso be supplied from the terminal +Sg of a source of screenunidirectional potential, not shown, through wide band choke coils 4|also constructed in accordance with the present invention. The outputvoltage i coupled through an inductor 4i and appears across a pair ofoutput terminals 5!, in turn coupled to any suitable utilizationcircuit, such as another amplifier stage or a transmission line.

Since the operation of a push-pull amplifier circuit such as that shownin Fig. 6 is well known, no detailed explanation of the operation of thecircuit will be undertaken. The utility of the wide band choke coils 4|in series in the anode voltage supply and in the screen electrodevoltage supplies to the two vacuum tubes VI and V2 will be set out. Inamplifiers of this design, where a great deal of output power isdesired, the amplitudes of the anode currents may reach large values ofcurrent. Since any resistance inserted in series in such acurrent-carrying lead will dissipate power, the ohmic resistance of sucha choke becomes of paramount importance. The choke of the presentinvention enables a connection which dissipates a minimum amount ofdirect current power, and at the same time offers a high resistance toradio frequencies.

Also, such high power push-pull amplifiers draw considerable screencurrent, and the screen isolation resistors necessary for the highfrequency components of current must dissipate large amounts of power.Such resistors and chokes which are required to dissipate large amountsof power are expensive to manufacture, and since the power dissipated insuch components i lost, the overall efficiency of the amplifier isconsequently reduced.

By utilizing the wide band choke coil of this invention, the screenisolation circuits are not required to dissipate large amounts of directcurrent power, and at the same time high impedance is offered to radiofrequency components, and the tendency toward instability andoscillation arising from screen current variation is greatly diminished.Although some of the advantages listed above can be obtained with achoke coil of conventional design if care is taken to insure that noneof the points of self-resonance coincide with harmonics of the frequencybeing transmitted, such precautions are unnecessary with the choke coilof the present invention. An even greater source of failure ofconventional choke coils arises in transmitters in which the frequencymust be changed from one frequency to another. If care is not taken toinsert the proper choke coil, particularly in power amplifier stages,the choke coil may be burned out or seriously damaged. In the choke coilof my invention, on the other hand, it is unnecessary to change thechoke coil because the points of self-resonance are so clamped by thelosses in the high resistance winding that no points of nearly zeroimpedance occur. Operation of transmitters which must transmit atdifferent frequencies is therefore simplified and the chance foraccidentally burning out an important component is eliminated.

I claim:

1. A wide band choke coil comprising a twoterminal network, a coil form,a high resistance winding wound in a single layer on said coil form, anda low resistance winding wound in a single layer over said highresistance winding in tightly coupled hut insulated relationship to saidhigh resistance winding the ends of said low resistance windingconstituting said terminals of said network, and at least one end ofsaid high resistance winding being unterminated.

2. A wide band two-terminal network radio frequency choke coilcomprising a coil form, a pair of terminals, a multifilar winding onsaid coil form, said multifilar winding having a plurality of conductorsin side by side relationship, one of said conductors of said multifilarwinding having a low resistance to direct current, the other con ductorsof said multifilar winding having a high resistance, the said lowresistance conductor having its ends connected to different ones of saidterminals, and the said high resistance conductors each having only oneend connected to one of said terminals the other ends being maintained.in insulated relationship and unterminated.

3. A wide band two-terminal network radio frequency choke coilcomprising a pair of tightly coupled windings, one of said windingshaving a low resistance and the other of said windings having a highresistance, a core of magnetic material within and extending throughoutthe length of both of said windings, a pair of terminals, said lowresistance winding having its ends connected to different ones of saidterminals, and said high resistance winding having only one endconnected to one of said terminal, the other end being open andunterminated.

4. In combination, an electron discharge device having an electrode, asource of unidirectional operating potential for said electrode, and aradio frequency choke coil connected between said electrode and saidsource, said choke coil including a pair of tightly coupled windings,one of said windings having a low resistance and the other of saidwindings having a high resistance, said low resistance winding beingconnected in series between said electrode and said source of operatingpotential, and said high resistance winding having only one endconnected to one end or" said low resistance winding the other end ofsaid high resistance winding being maintained in insulated relationshipto said low resistance winding and unterminated.

5. A wide band two-terminal radio frequency choke coil comprising, apair of terminals, a pair of tightly coupled windings, one of saidwindings having a low resistance and the other of said windings having ahigh resistance, said low resistance winding having its ends connectedto different ones of said terminals, and said high resistance windinghaving only one point thereon connected to said low resistance winding,the remainder of the high resistance winding being maintained ininsulated relationship with respect to said low resistance winding.

6. A choke coil as defined in claim 5 in which said low resistancewinding is composed of a plurality of pie sections, and said highresistance winding is composed of a plurality of pie sections, said highresistance sections being interposed between the said low resistancesections.

7. A choke coil as defined in claim 6 in which the said low resistancewinding is of copper wire, and the said high resistance winding is ofNichrome wire.

8. A choke coil as defined in claim 5 in which the said low resistancewinding is composed of a plurality of pie sections, and the said highresistance winding is composed of a plurality of pie sections, the saidlow resistance sections being alternated with the said high resistancesections and spaced closely thereto in tightly coupled inductiverelationship.

9. A choke coil as defined in claim 5 in which the said low resistancewinding is of copper wire and the said high resistance winding is ofNichrome wire.

References Gited in the file of this patent UNITED STATES PATENTS NumberName Date 1,761,110 Dijksterhuis et a1. June 3, 1930 1,791,236 DrakeFeb. 3, 1931 1,835,015 Crouse Dec. 8, 1931 1,837,413 Dcbson Dec. 22,1931 2,088,454 Whisk July 27, 1937 2,539,926 Rainwater Jan. 30, 1951FOREIGN PATENTS Number Country Date 21,646/29 Australia Mar. 25, 1925

